A.M.
Hi M.,
With regards to the food choice of your exchange students... I think it might be quite impossible for you to please everyone at the same time with food of there choice unless you really plan to go out of your way to make something different for everyone. I like might food hot and spicy too while my husband and daughter cant take it, so what i do is sprinkle some of the red pepper flakes (the ones that come with pizza) just on my plate or there's a big variety of hot sauces in the asian stores that can be used too. You should just give him that. While for the german girl, as long as she's not allergic to onions, if you grate it or chop it really fine to a point where she cant see it, she wont even know.
I cook a lot of indian and chinese food, so i know its almost impossible to take onions out of a dish.
I hope that helps.
Take care and take it easy.